Transmission system



w. M. BISHOP 2,272,735

TRANSMISSION SYSTEM Filed May '11, 1940 4 Sheets-Sheet 1 /N|/EN TOR W M. BISHOP frM liL ri l lL F. -IL F- L l;

Feb. 10, 1942.

Feb. 10, 1942. w. M. BISHOP TRANSMISSION SYSTEM Filed May 11, 1940 4 Sheets-Sheet 2 E OUALIZE I? lNVE/VTOR 14. M BISHOP Patented Feb. 10, 1942 TRAN SMIS SION SYSTEM Walter M. Bishop, Flushing, N.- Y., assignor to Bell Telephone Laboratories, Incorporated, ,New York, N. Y., a corporationofiNew York Application May 11, 1940, Serial No. 334,540

Claims. (-01. 1'79,.1;7Q)

This invention relates to transmission systems, and, more particularly, to a transmission system including one or more unattended repeater stations between attended stations or between the terminals of the system, and means and a method for controlling transmission levels along the system.

Transmission systems are known in which a plurality of repeater stations are located at geographically separated points along a transmission line between attended repeater stations or between the terminals of the line, and in which the power supply for the repeater stations is located at attended repeater stations only, or. at one terminal only of the system. Such a transmission system may include individual transmission lines for transmission in opposite directions respectively, such lines comprising pairs of insulated wires buried in the ground; or, two pairs of conductors in a buried ora suspended lead-covered cable; or, pairs of wires especially insulated for pole line construction; or, a pair of submarine cables. In such systems, variations in the temperature of the lines and their surroundings will cause variations in the transmission levels of the communications currents, for example, a voice frequency modulated carrier wave, being transmitted over the lines.

An object of the invention is to regulate, control or compensate for the elIects of temperature variation on the characteristics of the transmission system, such that the transmission level is maintained substantially constant.

In accordance with the invention, a transmission system comprising a pair of transmission paths or lines for transmitting electric wave energy in opposite directions, has a plurality of attended and unattended repeater stations at spaced intervals along the transmission lines. At each attended station, power supply is provided for the repeaters or amplifiers of the latter station and for the repeaters or amplifiers of the unattended stations between such attended station and the next attended station, or the power supply is provided at one terminal only of the system. Transmission level regulation or control means is provided at some of thestations-or, at each station, and/or at theterminals, and comprises resistance elements variable with temperature change and adapted for insertion in a gain control circuit for the repeaters or amplifiers. The means for varying the temperature and, therefore, the resistance of the variable resistance elements, responds to variations in the volt- .or current at the particular station, such power supply variations resulting from the variations in the temperature of the lines and their surroundings and, consequently, their impedance. The

invention also contemplates that resistanceqele- ,ments or devices variable with voltage thereacross or current therethrough may be used in the transmission level regulation means.

In a more specific aspect this invention contemplates a transmission system comprising a pair of transmission paths constituted by pairs of insulated wires buried in the ground by plowing .them therein with attendedand non-attended peater at .the unattended station. The heater element may be controlled by voltage or by current change in the power loop, or by change in the potential of a point in the loop with respect toa fixed potential (e. g., ground), and may be included in a network of positive and negative temperature coefiicient of resistance resistances,

or of varistor elements, such as copper oxide rectifiers or ballast lamps.

A, more complete understanding of this invention Will be derived from the detailed description that follows, taken in conjunction with the appended drawings, wherein:

Fig. 1 shows a transmission system in which this invention maybe embodied;

'Fig. 2 shows the repeaters or amplifiers at a station in the system of Fig. 1, in which transmission level regulation means in accordance with theiinvention are included;

Fig. 2A shows an alternative arrangement' ior the gain control means in-the ,B-path of-the repeatersof Fig. 2;

' Figs. 2B and 20 show alternative arrangements for the networks regulating the resistance-ofthe variable resistance element of the gain control circuits of Figs. 2 and 2A;

Fig. 3 shows a transmission level control circuit means included in the series or current path of the power supply;

Fig. 3Ashows an alternative for the levelconage 'orthe current of the powersupply voltage trol'circuit means-oi Fig. 3;

Fig. 4 shows level control circuit means at a repeater station, connected between the power supply phantom and ground;

Fig. 5 shows how the level control circuit means may be connected at the last repeater station between ground and a point on the plate voltage drop resistor near the neutral point of the power system; and

Fig. 6 shows characteristic curves for heater elements of different resistances that might be used in the bridge of the network WI of Fig. 2.

Fig. 1 shows a transmission system extending between geographically separated points or regions comprising a transmission path or line I!) for transmitting electric wave energy in one direction, for example, from west to east, and another transmission path or line H for transmitting electric wave energy in the opposite direction, that is, from east to west. Such electric wave energy might be communication or other signal energy superimposed on a wave of carrier frequency, and the transmission path could be adapted for the transmission of a single carrier wave in each direction, or for the transmission of a plurality of carrier wave frequencies distributed over a preassigned frequency band. For example, the system could be one adapted for 12-channel carrier operation utilizing a 12- kilocycle to 60-kilocycle frequency band. The transmission line may comprise pairs of insulated wires buried in the ground; or, two pairs of conductors in a buried or a suspended lead covered cable; or, pairs of wires especially insulated for pole line construction; or, a pair of submarine cables. The specific arrangement considered herein, however, is that of a transmission system comprising pairs of insulated wires buried in the ground, for example, by being plowed thereinto, and having one or more unattended repeater stations intermediate the terminal or attended station and also buried in the ground.

A plurality of attended repeater stations l2, [3, are provided at geographically separated points along the transmission line and a plurality of unattended repeater stations l4, l5, I6 at smaller separations are provided intermediate the attended station. If the transmission system is not a long one, the attended stations could be the terminal stations of the system, and only one unattended station might be required between them. At each repeater station there is an amplifier I! for amplifying electric wave energy incoming thereto and being transmitted in one direction, for example from west to east, I

and an amplifier 18 for amplifying electric wave energy incoming thereto and being transmitted in the opposite direction, that is, from east to west.

Fig. 2 shows the circuit arrangement for an unattended repeater station. The amplifiers l1, l8 are substantially identical and specific description of one is equally applicable to the other, like elements bearing corresponding identifying numerals. Ihe input and output terminals of amplifier H are coupled to the line 10 through repeating coils 24, and the corresponding terminals of amplifier 18 are coupled to line I! through repeating coil 25. Th amplifier I1 is a single stage feedback amplifier incorporating stabilized negative feedback as disclosed in H. S. Black Patent 2,102,671 issued December 21, 1937, the amplifying or mu-path of the amplifier being connected to the feedback path 26 through hybrid coils 21, 28 including thenetworks H, as

taught in the aforesaid Black patent, and in the pending H. S. Black application, Serial No. 114,390, filed December 5, 1936, allowed November 10, 1939 now Patent No. 2,209,955, issued August 6, 1940. The feedback path includes gain control means 29 and an equalizer network 30. The amplifying device, or tube, 31 may be a pentode having a cathode of the indirectly heated type. A network 32 may be connected in the cathode lead in accordance with D. D. Robertson Patent 1,994,486, issued March 19, 1935, to minimize any tendency for the amplifier to sing at some frequency outside of the transmission range. The heating current for the heater element of the amplifying devices and anode and screen grid potential are obtained over the transmission line, either from the terminals of the system or from an attended station, through the connections 33 and 34 coupling the mid-points of the line windings of the repeating coils 24, 25 in the lines H], II, respectively. The impedances in the connections 33, 34 are of high impedance to signal frequencies,

Figs. 1 and 2 show how anode and screen potential and cathode heater or filament heating current may be supplied to the unattended repeater stations from an attended station. The power supply may comprise the batteries 35, 36, the former having its positive terminal connected to the mid-point of the line winding of the repeating coil 24 at station I 2, and its negative terminal, together with the positive terminal of battery 35, connected to ground, the battery 36 having its negative terminal connected to the mid-point of the line winding of repeating coil 25 at station 12. As already noted with reference to Fig. 2, the mid-point of the line windings of the repeating coils in each line are interconnected by connections 33, 34, connection to the cathode heater being made through conductors 31, 38, and to the screen grids and anodes through conductors 39.

The power loop is completed through the connection 40 and suitable impedance 4|, for example, a resistor, shown in Fig. 1 as connecting the line windings of coils 24, 25 at attended station l3. Possible unnecessary power loss in the lines I0, ll between stations 16 and I3 could be obviated by having the power loop be completed through the line windings of the output coils 24, 25 at station 16, as indicated at 40'. Power supply for the repeaters or amplifiers (not shown) at attended station l2, which could be the same as those of repeaters l1, 18 of the unattended stations, may be obtained from the batteries 35, 36 or from a separate source of power. Although not shown, it is obvious that attended station I3 could be the source of power supply for unattended stations farther to the east along the transmission paths or lines.

The characteristics of the transmission lines of the system will vary with variations in the temperature of the lines and of their surroundings. The efiect of the variation in the frequency characteristics can be compensated or provided for in the equalizers associated with the feedback circuits of the repeaters. The efiect of the variation in the impedance or resistance of the lines on the transmission level of the cornmunications current can be compensated for by appropriate increase or decrease in the gain of the repeaters or amplifiers at the repeater stations.

Thermistors, i. e., devices that vary in impedance, or, specifically, resistance, with variation this fixed loss and the gain ;contr.ol.could becombinedto reducev anyundesirable-refiect thatjthe in their temperature,.and with ;;either a positive or a negative temperature coefficient of resistance, and .varistors, e., devices 'that -vary :in impedance,or,specifically; resistance, with vari- --ation-in the potential applied thereacross or the current flowing therethrough, .the resistance change withtemperature, voltage or current bethe fixed loss low. The resistance range of a silver sulphide thermistor in a vacuum tube, particularly, can be covered with the expenditure of a very'minute amount of power.

The use of-suchdevices involves at least two J" ance into that path to get the desired effect, and

a supply of power tothe heater elements for the thermistors, together with a control of such power that gives the desired relation between it and the temperature of the path to'be regulated.

The system described with reference to Figs. 1 and 2 is seen to be one of extreme simplicity, in that the. repeaters employ a single stage of amplification whose output may be below that necessary to operate known'thermistors. This would appear to eliminate direct pilot channel operation, except possibly on a manual basis at a low frequency. In such a system, furthermore, it might not be economical to provide extra line wires for any sort of resistance measuring arrangement, i. e., pilot wire operation, even if such an arrangement involved taking no appreciable extra power from the main power circuit. Inorder to keep the cost of the system, bothas to power consumption and equipment, to a minimum, it might be undesirable to add vacuum tube circuits to the system forgain regulation. It would appear, therefore, that entirely new means for and method of operating thermistor elementsare indicated forthe system described. 'The detailed description which follows hereinafter discloses how this need is met by the present invention.

In general, there are two ways in which thermistors may be used at a repeater station to cause a change in the net transmission of a circuit: to introduce a loss .in the direct transmission path, or to vary the losses in the c-circuit or path of the repeater or amplifier. Because of the power carrying limitations of thermistors such as silver sulphide, particularly direct current power, the devices should be isolated from the power circuit. Using thermistors in the direct transmission path would appear to require, therefore, the use of an extra repeatingcoil at the input of the amplifier, oran extra Winding on the present one. In eithercase, it might be necessary to terminate with the characteristic impedance at the point where the thermistor is inserted to prevent undue disturbance to the line impedance, which might introducemore fixed loss than could be tolerated. The introduction of the gain control thermistor into the ,B-path of the amplifier ofiers the least difi'iculty. Since the c-path'will, in general, require the introducti-on of some fixed loss .for other-reasons,

r mum value of the thermistor resistance.

' mistor gain control resistance-mighthave ion 2 the ,B-cir- :cuit propagation.

, It is necessary to increase: the losses in the B-pathin order to increase theggainof, therepeater when the transmissionlline and surrounding temperatures .areihigh, :and, in order to take advantage of :the greaterzreduction inthermistor resistance vdue to an increasing ambient temperature inincreasing the c-pathlosses, it isdesirable to Jshunt the thermistor across this circuit.

The simplified gain control circuit 29 of the amplifier of Fig. 2 comprises a fixedresistance R1 in .series with {the thermistor T, andthe value of which is large compared :to, the mini- R2 should be small compared tothe maximum:therresistance. The efiective resistance across the p-path varies, therefore, between R1 and R1+R2. By having these'resistances become ,efiective in something less thanrthefull. range of the transmission line temperatures, the effect upon the gain control circuit of thermistor vari ations due to manufacturing variations, aging and ambient temperature changes will be reduced. This gives the-gain control a tendency toward two-point working, i. e., high andlow. The arrangement 29 of Fig. 2A is an alternative for that of Fig. 2, and shows the resistances R1,-R2 and thermistor T connected in series with the ,B-path. The thermistor T andv its associated heater H are enclosed, as is wel1 known in the art, in an appropriate enclosure or oven 0.

Power for operating the thermistor T from the system of Fig. 1 powered by direct current supplied over the phantom, may be obtained in a number of ways. For example, there is a minimum potential drop at each repeater constituting power ordinarily wasted in adjusting the plate voltages to their desired values. Or, by supplying additional power "to the system, a small potential drop .in series with the power loop may be-made available at-each repeater. Or, a small current drain either across the power loop or from either'side thereof to earth may be utilized. In the case of the last repeater in the power loop, the current could be taken off of the plate voltage dropresistornear theneutral point of the power system. .I-Iow these sources may be utilized willnow be :describedin detail.

In Fig. 2 is shown an arrangement for using the power ordinarily lost in dropping the voltage across the phantom to the desired value for plate potential, or the drop from which plate potential isobtained for the last repeater in the power loop. The necessarynetwork is indicated in Fig. 1 by the dotted-line circle W, and a specific network W1 is shown, in Fig. 2.connected between points CC and D-D in the. connection '39 between-the plate of tube3! of repeater or amplifier ll and the connection 33. The network comprises two similar sections or portions 50, 60, the sections 50, 68 being associated respectively with the gain control .circuits of repeaters I 7, l8.

In each section 50, ,60, resistances T1 and maare ordinary resistances, either. independent of temperature or with a small positive temperature coeflicient such as is observed with iron wire.

Resistances m and T3 are thermistors with negative temperature coefiicients of resistances about like that of. silver sulphide, but .of some material, such as boron or uranium oxide, suitable ior use in direct current circuits. An auxiliary heater H1 for the thermistor T may be included for adjusting or biasing the thermistor T to preassigned extent or initial condition, variable resistance R3 being connected in shunt with the heater H1. Although the latter is not shown in the oven 0, it would be provided therein with the heater H. The thermistor T may be of silver sulphide, which is available with a maximum to minimum resistance ratio of 10 and can be made with a minimum resistance of one ohm, and be operated over this range by a total heater dissipation of about 21 milliwatts.

The arrangement shown in Fig. 2 for the network W1 has the advantage over the alternative networks We and W3 of Figs. 23 and 20, of giving a larger current ratio for any one heater, although not necessarily a larger total power ratio. Fig. 2 shows the heater control bridge B in but one heater circuit, that of heater H. A bridge circuit could be added to the second heater circuit, that of heater H1, or the circuits of networks W1, W2, We could be combined to obtain diiierent characteristics and adjustments as well as to enable the employment of the entire thermistor resistance ratio with the available plate current, if the latter should prove desirable.

In Fig. 2B, the sections 50', 60 comprise the gain control thermistor (T) heater H in series with heater circuit control resistor 7'5 similar to resistors r2, 1'3, and a parallel resistor re similar to resistors r1, r4; and in Fig. 2C, a resistor 1'7 similar to that of resistors T2, T3, T5, and the gain control thermistor (T) heater H".

If it is assumed that the transmission lines temperature range is between 0 and 21 C. and the resistors T1, r2, T3, T4 are located so that they will assume approximately the same temperatures as the lines, then it will be possible to get resistance ratios (r1=r4)/(T2= 3) of about 3:1. Fig. 6 shows the relation between TI=3TZ=T4 and I1 for different values of H for the arrangement of Fig. 2. The relation between the resistance 1'1 and E for both the balanced and unbalanced condition is also shown. As will .be evident from the curves, it is possible to get 10 milliwatts into the heater H when n equals approximately 2700 ohms and H equals 1000 ohms, at a current I1 of approximately 3.2 milliamperes. This will cause a voltage variation of about 26 volts across two such bridge circuits in series, most of which will appear on the plates of the tubes 3| since the plate circuits are inherently of a constant current nature. This voltage variation will be partly compensated for, however, by the wintersummer fluctuation of voltage in the power loop which will be in the opposite direction if the thermistor T is connected as in Fig. 2. The maximum impedance of the bridge and, therefore, the voltage and the voltage fluctuation, can be reduced by interchanging the values of the variable and the fixed arms of the bridge in Fig. 2, but the action of the thermistor T will be opposite for the same direction of temperature change. At the last repeater in the power loop, Where the circuit will be more nearly constant voltage in nature, the current 11 would be larger than indicated by Fig. 6, and a difierent heater, or a shunt across the 1000 ohm heater H, would be required. A feature of these thermistor power circuits when used in conjunction With the two-point operation mentioned above,

is the possibility of locating the temperature sensitive elements T2, T3 within the repeater service boxes or within the repeater containers.

Fig. 3 shows a circuit arrangement for taking power for the thermistor T from the power loop at a repeater station, in series with the connection 33. The necessary network is indicated in Fig. l by the dotted line circle X, and a specific network XI is shown in Fig. 3 connected between points E-E and FF in the power loop connection 33. The network XI comprises sections or portions 10, 80, the former being associated with the gain control circuit of repeater I1, and the latter being associated with the gain control circuit of repeater l8. Each of the sections 10, comprises a resistor R1 in the connection 33, this resistor being shunted by a series circuit comprising a varistor V, a heater or heating coil Hz for thermistor T, and a variable resistance rs. An alternative network X2 is shown by Fig. 3A wherein a ballast lamp L is connected in series with the connection 33 and has in shunt to it the thermistor heater H 2 and variable resistance T18. The varistor V may be of the copper-oxide type and, in connection with the arrangement of Fig. 3A, could also be included in series with the heater windings thereof. By operating the power loop input on a constant voltage basis, and by connecting the thermistor in series with the fl-circuit of the repeater as shown in Fig. 2A, it is possible to make use of the voltage variation in the power loop to operate this type of circuit on a fully automatic basis. By controlling the input voltage, it would be possible also to control the gain manually at the power source. The most sensitive arrangement as regards current variations would be to use a ballast lamp in series with the power loop and a varistor in series with the thermistor heater, the latter two both being in shunt with the ballast lamp. By working the ballast lamp at or near the first bend in its characteristic, a point may be found where a 5 per cent current variation (the current variation expected in a constant voltage powered loop as the temperature varies between 0 C. and 21 C.) which would produce the voltage change necessary to operate the varistor.

Figs. 4 and 5 show how the thermistor temperature regulating source may be obtained by taking off a small current either across the power loop, or from either side thereof to ground.

The latter alternative is shown in Fig. 1 and Fig. 4. Its necessary network is indicated in Fig. 1 by the circle Y in the dotted line connection to ground from connection 34, and a specific network YI is shown in Fig. 4 as comprising a varistor VI in series with two heaters or heating coils H3, H3, one for the thermistor of repeater I! and the other for that of repeater I8. The varistor VI may be of the copper oxide type, and should be connected to the loop at that repeater station where the voltage to ground can be varied 2 to 1 throughout the year, either automatically or manually.

The arrangement of Fig. 5 is adapted to be included at the last unattended repeater station in a single power loop. In Fig. 1 the necessary network would be associated with the repeater station 16 and with the connection 40. The connection 33 terminates at the end of impedance 4|, potential for the anodes of amplifiers IT, IS being obtained through the common point 9| connected to end 90 through the winding of relay 92. Connection 34 is connected to the other end 93 of impedance 4|, an intermediate point on which is connected to the-con-tact-kld of relay 92. One end oithe winding of Y asecond relay 95 is connected 'to armature fit of "relay 9-2, 'and its other end is connected to armature 91 and through the Y heaters H4, H4 and a resistance 93 to ground. "-Th-e cont-act 99 of '-relay tfi' is "connected to the series-connected heaters "H4 for the gainrontrol thermistors' in -the feedback circuits ofthe amplifiers 'I-l, l8. When'the'loop is under-going routine insulation tests andthe' power is off, relay 9'2 releases and groundis'removed from'theloop by the opening of contact between armature-Stand contact 95. RelayfiplaOes a short around the thermistor heatersHi by closure of armature-91 and contactsiwhen the current to ground exceeds a ,preassignedwaiue above which damage might result totth'eireaters. Resistanceilt reduces the -efiect of earth potentials which'might be superimposed on the phantom unbalance voltages.

Although this invention has been disclosed with reference to certain specific embodiments, it will be understood that they are believed at this'tim'eto be thepreferred'formsand that Lthe scope .of'the invention is not limited .thereto'but by "the. appended claims in the lighter theprior art.

VVhat'is claimed'is:

l. A transmission systemucomprisingia line for transmitting communication currentsiin one direction, .a line for transmitting =.communication currents in the reversedirection, arepeater-station along said paths comprising electron discharge device repeaters .for I amplifying said currents, each of said devices .comprisinga cathode, an input grid-and ananode, means forsupplying said station with anode pctential .-fr.om one terminal. only of said. system; and means-forregulating the gainsof .saidmepeaters .with variation in temperatureconditions along said lines comprising a feedback ..path.in eachirepeater and including anielement variable :in impedance -with temperature variation, and temperaturecontrolling meansiforsaid variablelelement includingea heater element the current through which; varies directly with. changes in.th'e'anodeepotential suppliecl'to the repeaters .oVer-the line.

2. A transmission systemcomprisinga trans mission ,path for transmitting -communication currents in one direction, "a'second' transmission path for transmitting communication currents in the .oppositerdirection, one or more repeater stations intermediate the :ends of :said transmissionlines with-separate amplifiers :at each stationfor amplification .of the L communication-fourrents .being transmitted in :each :direction, a phantom circuit loop in-cludingzsaid linestandsrepeater stations, a source of powersupply for said amplifiers-and included at one-'end of 'saidzl'oop, and means'for regulating the: gain of. said amplifiers with variation -iin temperature conditions along said linescomprising ta feedback path in each repeater and including an element variable in impedance with change in :itsztempera'ture, and=temperature-controlling means for said variable element including a theater element the current through which varies directly with changesin the-currentsupplied from said power source to the repeaters-over said loop.

.3. A transmission'system comprising a transmission path for transmitting communication currents in one direction,'arsecon'd transmission path for transmitting "communication currents in theopposite direction,-one orc'more repeater stations intermediate the ends of said transmission'lines with scparateamplifiers at each station-for amplification of the communication currents being transmitted in each directiona phantom circuitiloop including said lines-and repeater stations, a source of power supply 'for said amplifiers and included at oneend of said loop, and means forregulating thegainof said amplifiers with variation in temperature'conditions along said' lines comprising-a feedback path in each repeater 'andiincluding an element variable in impedance-withchange'in its temperature, and temperature controlling means forsaid variable element including a heater element "the current through which varies directly with changes in the potential between a point of fixed potential and -'a '.point'in said'loop at a repeater station.

4. --A transmission system comprising a trans: mission path for transmitting communication currents in-one direction, a second transmission path for transmitting communication currents in theopposite direction, a plurality of attended and unattended repeater stations intermediatethe ends of said transmission lines with separate amplifiers'ateachstation for amplification of the communicationcurrents being 'transmitted in-each direction, a phantom circuit=loop including said lines and repeater stations, a source of power'supply 'for said amplifiers and included at one end-of saidloop, and'meansfor regulating the gain of said amplifiers with variation in temperature conditions along --said lines comprising a feedback path in each repeater and including an elementvariable in impedance-width change'in its temperature, and temperaturecontrolling means for said variable element,-located at the'repeater'station at the end'of the phan tom circuit remote from the power supply and includinga heater element the current-through whichvaries directly with changes Iin the currentzsuppliedafrom said power-source to the repeaters'over said loop.

'5. A transmission system between geographically separated -pcints and buried in the'ground, comprising oppositelydirected transmission paths and'one or more geographically spaced repeaters therein for :amplifying the communication currents being transmittedihe power supply "for said repeaters being provided over a power loop comprising one of said paths from an attended repeaterzstation'to each unattended station intermediate attended stations, and back over-the other f said paths through each unattended-station tolsaidone attended station; and means at an unattended repeater station for regulating the transmission level of the communications currents, to compensate for changes in the characteristics' of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters 'at said unattended station, and means connected to said power loop for regulating the'temperature of 'said impedances and responsive "only to changes in the electrical condition of saidpower loop.

'6. A transmission system comprising oppositely directed transmission paths and one or, more geographically spaced repeaters therein for ampli'fying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended repeater ,station to each unattended station intermediate attend ed stations and back over the other of said paths through each unattended station to said one attended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, to compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedances and responsive to changes in the resistance of said power loop, in which the means for regulating the temperature of a said repeater impedance comprises a heater element in a network connected between the power loop and the plate electrode of the repeater, said network including a bridge of resistance elements.

'7. A transmission system comprising oppositely directed transmission paths and one or more geographically spaced repeaters therein for ampliiying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended repeater station to each unattended station intermediate attended stations and back over the other of said paths through each unattended station to said one attended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, t compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedances and responsive to changes in the resistance of said power loop, in which the means for regulating the temperature of the said repeater impedances at a repeater station comprises a heater element in each of a plurality of networks connected between the power loop and a plate of the repeater for amplification in one transmission path, each network including a bridge of resistance elements.

8. A transmission system comprising oppositely directed transmission paths and one or more geographically spaced repeaters therein for amplifying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended repeater station to each unattended station intermediate attended stations and back over the other of said paths through each unattended station to said one attended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, t compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedances and responsive to changes in the resistance of said power loop, in which the means for regulating the temperature of a said repeater impedance comprises a heater element in a network connected between the power loop and a plate electrode of the repeater, said network including a bridge of resistance elements, two opposite arms of which are of the type having a negative temperature coefficient of resistance.

9. A transmission system connecting geographically separated points and buried in the ground, comprising oppositely directed transmission paths and one or more geographically spaced repeaters therein for amplifying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended station to each unattended station intermediate it and the next attended station, and back over the other of said paths through each unattended station to said one at tended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, to compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedances and responsive to changes in the resistance of said power loop, said regulating means comprising a pair of series-connected resistors in series with the power loop, each resistor having in shunt thereto a heater for one of said impedances and a variable resistance element, said heater and resistance element being connected in series.

10. A transmission system connecting geographically separated points and buried in the ground, comprising oppositely directed transmission paths and one or more geographically spaced repeaters therein for amplifying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended station to each unattended station intermediate it and the next attended station, and back over the other of said paths through each unattended station to said one attended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, to compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedances and responsive to changes in the resistance of said power loop, said regulating means comprising a pair of series-connected ballast lamps in series with said loop, each of said lamps having connected in shunt therewith a heater element for one of said impedances.

11. A transmission system connecting geographically separated points and buried in the ground, comprising oppositely directed transmission paths and one or more geographically spaced repeaters therein for amplifying the communication currents being transmitted, the power supply for said repeaters being provided over a power loop comprising one of said paths from an attended station to each unattended station intermediate it and the next attended station, and back over the other of said paths through each unattended station to said one attended station; and means at an unattended repeater station for regulating the transmission level of the communication currents, to compensate for changes in the characteristics of the transmission paths with change in their temperature and that of their surroundings, said means comprising temperature-controlled variable impedances in the repeaters at said unattended station, and means regulating the temperature of said impedance-s and responsive to changes in the resistance of said power loop, said regulating means comprising a pair of series-connected heaters for the impedances of the repeaters at the unattended station, said heaters being connected in series with a variable resistance element and between a point of fixed potential and a point on said loop at said unattended repeater station.

12. A transmission system comprising a transmission line, an amplifier in said line comprising an electron discharge device having a cathode, an input electrode, an anode, and a cathode-heating filament, energizing power for said anode and said filament being supplied over said line, a feedback connection between the output and input of said amplifier, temperature-responsive resistance means in said feedback connection for regulating the feedback in and the gain of said amplifier, and means for altering the temperature of said resistance means in accordance with temperature change in said line, said temperature-altering means being responsive only to a change in an electrical characteristic of the energizing power supplied over said line.

13. A transmission system comprising a transmission path for communication currents, a repeater station in said path, means at one end of said path for supplying power to said repeater station over said path, a communication cunrents path through said repeater at said repeater station, a power path around said repeater at said repeater station, and gain control means at said station, said means including an impedance device variable in impedance with temperature, and a temperature-varying device connected to said power path and responsive only to a change in the electrical condition of said power path for varying the temperature of said impedance device with change in the electrical condition of said power path.

14. A transmission system comprising a transmission path, an amplifier in said path, said amplifier comprising an electron discharge device having an anode, a power supply at one end of said path for said amplifier, a power path around said amplifier, the potential for the anode of said device being supplied over said paths, and means to regulate the gain of said amplifier with variations in the line characteristics because of changes in the temperature of the transmission path and its surroundings, said means including a current responsive device connected to said anode and said power path.

15. A transmission system comprising a transmission path, an amplifier in said path for communication currents, said amplifier comprising an electron discharge device having an anode, a cathode, and a cathode-heater filament, a power supply for said amplifier at one end of said path, the potential for said anode and the current for said filament being supplied over said path, a feedback connection for said amplifier between the output and input circuits of said amplifier, a temperature-controlled variable impedance in said feedback connection arranged so that its impedance change alters the amount of feedback, and temperature-varying means for said variable impedance, connected to said path but independent of the output of said amplifier, said means comprising a heater element the current through which varies with change in the electrical condition of the transmission path with respect to the power supplied thereover.

WALTER M. BISHOP. 

